1. Field of the Invention
The present invention relates to apparatus for use in the photocatalytic treatment of liquids and more particularly to apparatus for and a method of decontaminating a contaminated fluid by utilizing photocatalytic particles.
2. Description of the Prior Art
A relatively new technology uses semiconductor powders to carry out photocatalytic processes. Irradiation of the semiconductor with light of energy equal to or greater than the band gap results in the creation of electrons in the semiconductor conduction band and holes in the valence band followed by trapping of the separated charges in shallow traps at the semiconductor powder surface. The injection of these electrons and holes into the fluid region surrounding the semiconductor particles' surface causes electrochemical modification of substances within this region. Such technology has been used in at least the following photocatalytic processes: the photo-Kolbe reaction in which acetic acid is decomposed to methane and carbon dioxide; the photosynthesis of amino acids from methane-ammonia-water mixtures; the decomposition of adipic acid to carbon dioxide, butane, and valeric acid; the production of hydrogen from several aliphatic and aromatic compounds (including fossil fuels) with water; the degradation of chlorinated hydrocarbons to carbon dioxide and hydrochloric acid; the oxidation of cyanide; the sterilization of aqueous microbial cells; and the deposition of metals from their aqueous ions.
As is well known, catalytic action results when a catalytic agent reduces the activation energy required to drive a chemical reaction to completion. In ordinary heterogeneous catalysis, the activation energy, Ea, is provided by heat and the catalyst reduces the amount of heat required. Hence, the catalyst permits driving the chemical reaction at a faster rate at a given temperature or lowers the temperature at which a given rate occurs. In contrast, in photocatalysis, Ea is provided by the photon energy of the incident band-gap light. Light has a very high free energy content, and it can be converted into high energy electron excitation when absorbed by semiconductors. Thus, optically excited semiconductors can drive chemical reactions even at room temperature by providing Ea in the form of high energy electrons and holes.
Photocatalysis is distinguishable from ordinary heterogeneous catalysis in that it employs visible and ultraviolet (UV) radiation to facilitate chemical reactions rather than thermal energy (i.e., heat). Although the infrared (IR) part of the spectrum is also electromagnetic radiation, its absorption by matter normally results in heating. Absorption of IR radiation will result in the heating of the catalyst or chemical reactants. In ordinary catalysis, thermal energy derived from IR irradiation, direct heating or even microwave absorption manifests itself as an elevated temperature (increased energy of translational, rotational, and vibrational modes) of the chemical reactants and the catalyst for the provision of the reaction's activation energy. The optical properties (i.e., the ability of radiation to effect electronic excitations) of the catalyst are not germane. In ordinary catalysis the catalyst may even be a semiconducting material. However, this is an incidental property, and no particular use of its optical properties is made. The ordinary catalyst is optically passive, and only provides an adsorbing surface for diminishing the activation energy of reactants.
As a result, the role played by IR, visible, and UV light in ordinary catalysis and photocatalysis is fundamentally different. In distinction from ordinary catalysis, in heterogeneous photocatalysis, the catalyst's optical properties are paramount. The photocatalyst must be a semiconductor. By absorption of band-gap light, electron and hole charge carrier pairs are produced within the photocatalyst particles. These charge carriers then perform redox reactions with the chemical species. Ordinary catalytic properties, as described above, may also be a feature of the photocatalytic process. Additionally, ordinary thermal processes may also play a secondary role in reaction kinetics (e.g., absorption of any wavelength light could result in some system heating). However, the distinguishing feature of photocatalytic reactions is that the activation energy of reaction results primarily from optical processes and the subsequent generation and transfer of electron holes (i.e., redox chemistry) rather than simple heating.
Some of the exemplary literature describing experiments utilizing such technology are: "Photocatalytic Reactions of Hydrocarbons and Fossil Fuels with Water. Hydrogen Production and Oxidation", by K. Hashimoto, T. Kawai, and T. Sakata, J. Phys. Chem., Vol. 88, No. 18, pp. 4083-4088, 1984; "Solar Photoassisted Catalytic Decomposition of the Chlorinated Hydrocarbons Trichloroethylene and Trichloromethane", by S. Ahmed and D. Ollis, Solar Energy, Vol. 32, No. 5, pp. 597-601, 1984; "Heterogeneous Photocatalytic Decomposition of Benzoic Acid and Adipic Acid on Platinized TiO.sub.2 ", by M. Fujihira, Y. Satoh and T. Osa, Nature, Vol. 293, pp. 206-208, 1981; "Powder Layer Photoelectrochemical Structure", by R. E. Hetrick, J. App. Phys., Vol. 58, No. 3, pp. 1397-1399, 1985; "Solar Assisted Oxidation of Toxic Cyanide", Langley Research Center, Hampton, Virginia, NASA Tech Briefs, p. 106, Spring 1985; "Photocatalytic Decomposition of Water and Acetic Acid Using a Powder-Layer Photoelectrochemical Structure", by R. E. Hetrick, App. Phys. Comm., Vol. 5, No. 3, pp. 177-187, 1985; "Photoelectrochemical Sterilization of Microbial Cells By Semiconductor Powders", by T. Matsunaga, R. Tomoda, T. Nakajima, and H. Wake, FEMS Microbiology Letters, Vol. 29, pp. 211-214, 1985; "Photocatalytic Deposition of Metal Ions Onto TiO.sub.2 Powder", by K. Tanaka, K. Harada, and S. Murata, Solar Energy, Vol. 36, No. 2, pp. 159-161, 1986.
Photocatalytic processes have also been used to decompose into relatively harmless products, the following chemicals: trichloroethylene, catechol, acetic acid, benzonitrile, phenol, caprolactam, acetone, methanol, urea, dichloromethane, chloroform, carbon tetrachloride, trichloromethane, 4-chlorophenol, pentachlorophenol, polychlorinated biphenyl, dioxins, p-dichlorobenzene, 1,1-dibromoethane, and 1,2-dibromoethane. Photoelectrochemical processes employing irradiated semiconductor slurries have been investigated for the removal of metals by photoeleotrodeposition; for example chromium(VI), copper(II), cadmium, lead, arsenic, and mercury.
It is believed that all present systems utilizing the technology maintain the chemical compounds to be modified in a gas mixture, gas solution, a gas/liquid mixture, a liquid, or another fluid for the most effective contact between the chemical compound and the semiconductor powder. In most of the systems, the semiconductor powder itself is also suspended and mixed in the gas or liquid and is maintained in such suspended and mixed condition by bubbling a gas through a liquid, constantly stirring the fluid with a magnetic stirrer, for example, or continuously circulating the fluid with a pump. A problem with suspending and mixing the semiconductor powder in a gas or liquid is that some means must be utilized to maintain the semiconductor powder in a suspended and mixed condition and that the semiconductor powder must at some time be segregated from the modified chemical compound, especially if the semiconductor powder is to be reused. In another system, the photocatalyst can be immobilized onto a substrate. If the photocatalyst powder particles are small enough, they can be made to effectively adhere to the substrate by relatively weak forces. Another method of immobilization is to cause chemical binding of the photocatalyst to the substrate. Although immobilizing the photocatalyst solves the problem of powder segregation after completion of chemical reactions, the immobilized system can suffer from inefficient light usage due to mass transfer problems or the photocatalyst can be poisoned, thereby necessitating the replacement of the immobilizing matrix. It also appears that the known systems prior to the development of the invention in the co-pending patent application carry out photocatalytic processes on a batch or intermittent basis for modifying relatively small amounts of chemical substances.
One of the major barriers to implementing photocatalytic technology on a commercial scale has been the lack of a technique for the efficient separation of the semiconductor powder from the slurry reaction mixture due to the small size of the particles, which are usually in powder form and of sub-micron diameters. Use of conventional filters to separate the semiconductor powder from the slurry reaction mixture causes the powder to quickly cake over the filter and eventually clog the filter to the point where the filter fails to function. Another technique that has been used to separate the semiconductor powder from the slurry reaction mixture is to centrifuge the slurry, however, this process usually does not effect complete separation, and centrifuge equipment is cumbersome to use and is expensive. Also, the centrifuge process may require that the operation of the photocatalytic process be disrupted in a non-continuous operation. Consequently, in any commercial, large scale, substantially continuous photocatalytic operation in which the semiconductor powders are suspended in the fluid containing the substances to be decomposed, there is a need for an effective and efficient particle separation system that will segregate the semiconductor powder from the fluid after the decomposition has been achieved.